The present invention relates to subterranean treatment fluids. More particularly, the present invention relates to methods and compositions for enhancing guar hydration rates and performing guar derivatization reactions.
Treatment fluids are used in a variety of operations and treatments performed in oil and gas wells. Such operations and treatments include, but are not limited to, production stimulation operations, such as fracturing, and well completion operations, such as hydraulic fracturing, gravel packing and frac packing.
In hydraulic fracturing, a type of treatment fluid, referred to in the art as a fracturing fluid, is pumped through a well bore into a subterranean zone to be stimulated at a rate and pressure such that fractures are formed or enhanced in a desired subterranean zone. The fracturing fluid is generally a gel, emulsion, or foam that may comprise a particulate material often referred to as proppant. When used, proppant is deposited in the fracture and functions, inter alia, to hold the fracture open while maintaining conductive channels through which such produced fluids can flow upon completion of the fracturing treatment and release of the attendant hydraulic pressure.
Gravel packing treatments are used, inter alia, to reduce the migration of unconsolidated formation particulates into the well bore. In gravel packing operations, particulates, referred to in the art as gravel are suspended in a treatment fluid, which may be viscosified, and the treatment fluid is pumped into a well bore in which the gravel pack is to be placed. As the particulates are placed in the zone, the treatment fluid leaks off into the subterranean zone and/or is returned to the surface. The resultant gravel pack acts as a filter to separate formation solids from produced fluids while permitting the produced fluids to flow into and through the well bore. While screenless gravel packing operations are becoming more common, traditional gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel designed to prevent the passage of formation particulates through the pack with produced fluids, wherein the well bore may be oriented from vertical to horizontal and may extend from hundreds to thousands of feet.
In some situations the processes of hydraulic fracturing and gravel packing are combined into a single treatment to provide a stimulated production and an annular gravel pack to prevent formation sand production. Such treatments are often referred to as “frac pack” operations. In some cases the treatments are completed with a gravel pack screen assembly in place with the hydraulic fracturing treatment being pumped through the annular space between the casing and screen. In this situation the hydraulic fracturing treatment ends in a screen out condition creating an annular gravel pack between the screen and casing. This allows both the hydraulic fracturing treatment and gravel pack to be placed in a single operation. In other cases the fracturing treatment may be performed prior to installing the screen and placing a gravel pack.
A variety of methods are used to create the viscosified treatment fluids typically used in subterranean operation. Generally, a polysaccharide or synthetic polymer gelling agent is used to impart viscosity to the treatment fluid to, among other things, enhance proppant or gravel transport and reduce fluid loss from the treatment fluid into the formation. Frequently, a crosslinking agent, such as a metal ion with organic or inorganic counterion, organometallic or organic compound, is also added to further enhance the viscosity of the fluid by coupling, or “crosslinking,” polymer molecules. The treatment fluid may also include one or more of a variety of well-known additives, such as gel stabilizers, fluid loss control agents, clay stabilizers, bactericides, and the like.
Guar gum and guar derivatives are commonly used in the oilfield to manufacture treatment fluids. Guar gum is typically prepared by mechanically and/or chemically treating guar beans to liberate the guar seed endosperm, or “guar splits,” from the beans. Guar splits primarily comprise a polymannose backbone with galactose side chains and mannose, and contain a fair concentration of contaminates, such as cellulose, protein, and glycolipids. The guar splits are generally treated under high pressures and temperatures with chemicals, after which they are subjected to multiple washings to remove impurities and salts (which are byproducts of some of the treatments) from the splits. The treated and washed splits are then ground and dried to yield derivatized guars.
The guar powders are typically dispersed into a water-based fluid, such as a 2% KCl solution, and allowed time to hydrate. This dispersion may be accomplished by adding the powdered guar directly to water, or by first creating a liquid slurry, or liquid gel concentrate (“LGC”), of the powder in a non-hydrating solvent, such as diesel. After hydration, the guar fluid is significantly higher in viscosity, making it possible to transport high-density propping agents through pumping equipment and into a subterranean formation.
Despite their widespread use, guar-based treatment fluids do have some technical disadvantages. For example, the time necessary for complete hydration and/or viscosity generation for guar-based fluids may take several minutes. This can be particularly inconvenient during on-the-fly fracturing applications. In order to successfully use guar-based fluids in a continuous fashion requires the use of large volume (i.e., long residence time) holding tanks to permit the hydration of the guar gum. In addition to requiring additional equipment at the well site, this large holding volume limits the ability to change fluid formulations in response to real-time pressure changes that may be measured during the fracturing treatment. Even the derivatization of the guar can prove costly and/or inconvenient, as the derivatization process typically requires large reactors capable of handling dry materials to treat the guar splits, increasing the equipment expense necessary for creating the guar-based fluids.